In the reel-lay method of laying an offshore pipeline, the pipeline is provided from a reel or spool on a vessel, towards a lay-tower designed to direct the pipeline down through the lay-tower and through a moonpool in the vessel and into the sea for laying. The lay-tower may be in a ‘vertical’ position relative to the general plane of the vessel, or at an angle thereto, generally by rotation of the lay-tower at or near a point on the vessel.
At the top of the lay-tower is a circular aligner, typically a large grooved wheel. Thus, the pipeline extends from the reel to the top of the tower in an ‘oblique’ direction compared to the ship level and the lay-tower, over the lay-tower aligner, and down through the lay tower. For the purposes of the present invention, the portion of the pipeline extending from the aligner and down through the lay-tower shall be defined as a ‘vertical’ part of the pipeline, independent of the angle of the lay-tower relative to the vessel.
During offshore installation, there may be a requirement to install into the pipeline a device or apparatus, for example an In-Line-Tee (or ILT or ‘T-piece’), a manifold, a PipeLine End Termination (PLET) also known as FlowLine End Termination (FLET), or an Abandonment and Recovery Head (‘A&R’ Head) to perform an abandonment and recovery ‘A&R’ operation by the addition of the Abandonment and Recovery Head (‘A&R’ Head) at the end of the pipeline known in the art. For the purposes of the present invention, any such apparatus, pipe, etc. to be installed ‘in-line’ shall be defined as an In-Line Structure or ILS.
For the installation of such an In-Line Structure (ILS), the unreeling of the pipeline has to be interrupted, the pipeline cut in two parts, adjustment of the space between the cut ends optionally made to match the space required for the In-Line Structure (ILS), the In-Line Structure (ILS) installed between the pipeline's cut ends, before the unreeling of the pipeline can start again.
Where the pipeline is relatively thin or single layered, the change of direction of the pipeline from its oblique part to its vertical part around the lay-tower aligner should not cause any significant damage to the pipeline during laying, such that any movement of the pipeline required to create the space to insert the In-Line Structure (ILS), and the re-start of reel-laying thereafter, is immediately possible. Such an arrangement is shown in for example U.S. Pat. No. 6,733,208, or WO2006/085739A.
However, there are some pipelines where it is desired to have them flooded and typically pressurised during reel-laying to minimise or avoid damage to the pipeline as it changes direction of the pipeline from its oblique part to its vertical part around the lay-tower aligner. The flooding and typical pressurisation of the pipeline with a fluid such as water or MEG, maintains an internal pressure within the pipeline in a manner known in art.
Such pipelines include Mechanically Lined Pipelines (MLP). Pipelines for use in the subsea conveying of fluids such as gas or crude oil are often exposed to the corrosive effects of these fluids which, unless the pipeline is protected by some material resistant to their effect or corrosion, can cause damage to the pipeline and even pipeline failure, which is difficult and expensive to remedy. Corrosion Resistant Alloys (CRA), such as Inconel™ 625, are known to provide resistance to corrosion in extreme environments such as in oil and gas pipelines. To avoid having to unnecessarily provide the entirety of the pipeline as relatively high cost Corrosion Resistant Alloy (CRA), it is known to provide bimetallic pipeline in which an outer, typically carbon steel pipeline is provided as a host pipe, the inner surface of which is protected by a layer of a Corrosion Resistant Alloy (CRA).
It is known to provide bimetallic pipeline by cladding the inner surface of the carbon steel pipeline with a metallurgically bonded Corrosion Resistant Alloy (CRA) layer, or even a weld overlay. However an alternative method, which has in some cases a cost benefit, of producing bimetallic pipeline is to add a liner to the inner surface of the carbon steel pipeline made from Corrosion Resistant Alloy (CRA) or other material. The lining process involves the creation of a mechanical bond between the liner and the pipeline by inserting the liner into a length of the pipeline and hydraulically or mechanically expanding the pipeline and liner together, such that the liner undergoes a plastic deformation while the outer pipeline undergoes an elastic or plastic deformation. Upon relaxation of the expansion force or pressure, an interference contact stress or interference fit is produced at the interface between the liner and the pipe, causing the liner to become mechanically bonded to the internal surface of the pipe.
While a Mechanically Lined Pipeline (MLP) may be preferable from a pipeline manufacturing and cost-effectiveness perspective, a known problem is that, in pipeline construction methodologies that involve reeling the pipeline onto and off a storage spool during production and laying, forces imparted on the pipeline during the reeling process by bending of the pipeline can cause the internal liner to wrinkle. In reeling a Mechanically Lined Pipeline (MLP), significant bending strain is imparted to the pipeline as it is wound onto and unwound from a storage reel which can result in a significant amount of wrinkling in the Corrosion Resistant Alloy (CRA) liner. The wrinkling mechanism may be as a result of, among other things, the bending stress itself, ovalisation of the pipeline cross section, or differential longitudinal compression and strains on the liner along the curvature of the bend due to the periodic circumferential fixation of the liner in the region of the joining welds. This wrinkling is an undesirable result, as the wrinkles can cause mechanical and material issues with the liner (such as embrittlement), as well as causing significant problems within the pipeline during and after the pipeline is commissioned and is in use, which can lead to the failure of the pipeline before the end of its serviceable term.
To avoid wrinkling a reeled Mechanically Lined Pipeline (MLP), it is known that a sufficiently thick Corrosion Resistant Alloy (CRA) or other liner can reduce the likelihood or magnitude of wrinkling, or avoid wrinkling completely, when the pipeline is bent or deformed during laying. Indeed, due to this phenomenon, Corrosion Resistant Alloy (CRA) or lined pipeline is generally chosen for a specific project having a liner thickness sufficient to avoid wrinkling due to the laying process. In the case of corrosion resistant liners, the fundamental thickness of the Corrosion Resistant Alloy (CRA) liner that is required is that which is sufficient to protect against corrosion over the serviceable life of the pipeline, and is dependent on the conditions in which the pipeline is to operate. In many cases the liner thickness that is used is greater than that needed to withstand chemical attack over the serviceable life of the pipeline, or that needed to successfully perform the function of the liner. For example, in the context of reeling a Mechanically Lined Pipeline (MLP) onto and off of a spool, WO 2011/048430A discloses two methodologies for calculating a minimum liner thickness necessary to avert wrinkling during reeling of a Mechanically Lined Pipeline (MLP).
Given that the cost per unit weight of Corrosion Resistant Alloy (CRA) and other liner materials can be very high, the cost of the liner material can become a significant component of the fixed material costs of the pipeline. Thus while using a sufficiently thick liner can reduce wrinkling, it can also lead to the use of a large amount of an expensive liner material. Where a thicker liner than is necessary to avoid corrosion is still used to reduce wrinkling, the amount of liner material used can be far in excess of that needed for the pipeline to function properly and, e.g. to protect against chemical attack during use.
Other possible solutions to avoid wrinkling a reeled Mechanically Lined Pipeline (MLP) during reel-laying are described in WO2008/072970A, WO2011/124919A, WO2011/051221A, WO2011/051218, and WO2010/010390. These include pressurising the entire length of the pipeline as it is reeled onto a storage reel, and later pressurising the entire length of the pipeline as it is unwound from the storage reel, straightened and laid, after which it is depressurised. The pressurisation appears to help prevent the liner from wrinkling due to bending stresses in the pipeline on reeling and unreeling.
Where a pipeline is flooded and pressurised, there is also a need to ensure the safety of the operators while the pipeline is cut and to guarantee the pressurisation of the pipeline during any movement of the pipeline, as well as when the unreeling starts again.